The present invention relates to a drydocking method and apparatus for raising a vessel from a body of water. More specifically, the present invention concerns a drydocking method and apparatus including a moment resisting arrangement for maintaining a platform in a substantially horizontal plane while raising and lowering the vessel supported thereon.
It is known in the shipping art that repairs on a floating vessel are facilitated if the vessel can be lifted from a body of water. Various devices for vertically moving or "drydocking" a vessel are known. However, these drydocking devices have several disadvantages.
One known drydocking device is a graving dock which includes a basin adjacent to a body of water. The basin is capable of being filled with water through a gate which separates the basin and the body of water. In operation, the vessel floats through the gate and, after closing the gate, water is pumped from the basin so that the vessel rests on the floor of the basin. The vessel is launched by refilling the basin and floating the vessel into the adjacent body of water. The graving dock, however, requires large quantities of water to be pumped into and out of the entire basin to drydock the vessel, regardless of the size of the vessel. Further, the graving dock occupies large areas of valuable waterfront property which may be put to a more beneficial use.
Elevator-type drydocks are also known in the art. These drydocks include mechanical devices such as winches or hydraulic rams for lifting or lowering a vessel. Elevator drydocks, however, require mechanical devices with large lifting capacities which must be synchronized so as not to overload a particular device or tilt the vessel.
Wing-walled floating drydocks are also known. These drydocks include a pontoon having bouyant vertical walls (wing walls) on longitudinal sides of the pontoon. The pontoon is divided into numerous internal compartments for stabilizing the pontoon while lifting a vessel situated between the wing walls. The wing-walled floating drydock, however, requires complex piping and valving mechanisms for simultaneously flooding or evacuating each compartment within the pontoon.
Another drydocking device includes a submersible barge situated between two fixed structures, such as a pier and a piling. The barge has first and second pulleys located on a side of the barge adjacent to the pier and third and fourth pulleys located on a side of the barge adjacent to the piling. A first chain extends from a top of the pier around the first and third pulleys to a base of the piling. A second chain extends from a top of the piling and around the fourth and second pulleys to a base of the pier. In operation, the vessel is positioned between the two fixed structures and above the submerged barge. The barge is evacuated to raise the vessel thereon while the two chains maintain the barge in a horizontal plane. Such a drydocking arrangement, however, requires two fixed structures which increases the expense of the drydocking device and prevents ready access to the barge from two sides. A device of this type is disclosed in U.S. Pat. No. 3,559,606 issued to Gregory.
A common problem associated with both wing-walled floating drydocks and submersible barges is the maintainance of the drydocking device in a substantially horizontal plane while raising or lowering the vessel supported on the drydocking device. This problem of stability becomes exacerbated if the center of gravity of the vessel is not aligned with the center of bouyancy of the drydocking device, i.e., the vessel is eccentrically positioned with respect to the center of bouyancy of the device. Eccentrically positioned vessels create capsizing moments tending to rotate the device out of the horizontal plane. The above-described known devices maintain stability with wing-walls or with chains extending between two fixed structures, but in doing so limit vessel access to the drydocking device to the ends of the device between the wing walls or the fixed structures.
An additional problem associated with the stability of wing-walled floating drydocks and submersible barges is that the positioning of the vessel on these known drydocking devices is critical. The positioning of the vessel on these devices must be closely monitored so that the moment created by an eccentrically positioned vessel does not exceed a moment resisting tolerance of the wing-walls or the chains. Therefore, the eccentricity of a vessel positioned on known drydocking devices must be minimized.
The present invention concerns drydocking method and apparatus including a moment resisting arrangement which maintains the drydocking device in a substantially horizontal plane without impeding vessel access to the drydocking device and without closely monitoring the positioning of the vessel on the device. In order to eliminate the numerous internal compartments and the associated complex valving and piping arrangements employed in wing-walled floating drydocks, the present invention includes a submersible pontoon having at least one internal chamber into which air and water are introduced. The pontoon has an arrangement for selectively introducing air or water into the chamber for raising and lowering the pontoon so that there is no need for mechanical lifting devices as in elevator drydocks. At least one portion of a first side of the pontoon is positioned adjacent to a single fixed structure so that the drydocking device with the moment resisting arrangement of the present invention does not occupy an extensive area of waterfront property.
The moment resisting arrangement includes a cable secured to the fixed structure and extending under and around a first sheave positioned adjacent to the first side of the pontoon. The cable extends across the pontoon and passes over a second sheave positioned adjacent to a second side of the pontoon opposite from the first side. The cable is anchored to an anchoring mechanism positioned beneath the pontoon so as not to limit vessel access to pontoon to only the ends of the device. The cable extended between the fixed structure and the anchoring mechanism has a moment resisting capacity sufficient to resist external moments due to eccentrically positioned vessels which tend to rotate the first side of the pontoon downwardly.
The moment resisting arrangement also includes an arrangement for creating an induced moment which tends to rotate the first side of the pontoon downwardly. The induced moment resists external moments due to an eccentric vessel which tends to rotate the second side of the pontoon downwardly. Preferably, the arrangement for creating an induced moment includes a permanent void tank(s) and a permanent ballast tank(s), both of which are sealed tanks positioned within the pontoon.
In operation, the pontoon is positioned between the fixed structure and the anchoring arrangement. Air and water are selectively introduced into the chamber within the pontoon in order to raise and lower the pontoon. If the center of gravity of a vessel on the pontoon does not align with the center of bouyancy of the pontoon, then a resulting moment tending to rotate the pontoon from a substantially horizontal plane is resisted by either the cable extending between the fixed structure and the anchoring arrangement or by the induced moment created by the permanent void tank(s) and/or the permanent ballast tank(s).
Many objects and advantages derived from the present invention will become apparent to those skilled in the art from this specification.